Material for electrophotographic purposes



3,148,057 ELECTROPHOTOGRAPHIC MATERIAL FOR PURPOSES Siegfried Raether, Hofheim, Taunus, Germany, assignor,

The electrophotographic process comprises electrostatically charging, in the absence of actinic light, a photoelectrically conductive insulator layer, which contains or consists of a ph otosemiconductor compound, and the subsequent imagewise exposure to light of the charged surface, whereby the charge leaks away in the parts affected by the light. The latent electrostatic image thus produced is made visible by dusting over with a powder and is then fixed.

'For the preparation of photoelectrically conductive coatings, the use of organicand inorganic photosemiconductors has already been proposed. These photosemiconductors must, however, be applied in an extremely finely divided form, with or without a binder, in a coating of specific thickness to a supporting material.

The present invention relates to a material for electrophotographic purposes consisting of a'metal support and a photoelectrically conductive insulator layer in which the support consists of one or more metals susceptible of anodization, or is coated on the surface with such a metal, and in which the photoelectrically conductive insulator coating comprises or consists of an oxide layer produced by anodization of the metal surface.

The material of the present invention is distinguished from electrophotographic materials hitherto known in the novel composition of the photoelectrically conductive insulator coating. The usual practice, whereby a support is coated with a photosemiconductor substance and which requires special processes, special binders and additives or a special vaporization process, is eliminated. Further desirable,properties are the hardness and excellent adhesion of the metal oxides to the surface of the support, the high dark resistance, the low production costs and the absence of a binder, and hence of a coating which must be removed in the image-free parts before the material can be used for offset printing.

Suitable metals for use in the invention include all those which are capable of being anodized.- Particularly favorable results can be obtained with aluminum, tantalum and titanium.

Instead of the expression anodization the term anodical oxidation" could be used for the purposes of the present invention.

The anodizing or anodical oxidizing process is carried out in accordance with known methods, e.g., by the immersion of a metal foil, as one electrode, and any type of ordinarily available counter-electrode in an electrolyte solution containing, for example, citric acid, sulfuric acid or boric acid, followed by the application of a voltage of about 25 to 250 volts, preferably 150 to 250 volts, between the'tw o electrodes. After the current has been turned off, the plate is dried at elevated temperature.

Other anodizing methods are described in the following texts:

A. Gunther Schulze and H. Betz: Elektrolytkondensatoren, Verlag M. Krayn, 1937;

M. Schenk: Werkstoff Aluminium und seine anodische Oxydation, A. Franke AG., Verlag Bern, 1948.

i The metals are advantageously used in the form of selfsupporting foils or films which are produced in the usual way, e.g., by casting or by rolling. However, the metals may also be applied in thin sheets, either before or after ited States Patent 3148057 Patented Sept. 8, 1964 the oxidation process, to a supporting material, e.g., ceramic ware, other metals, or plastic foils.

Also. plastic foils to which the metals are laminated or plastic materials which have been provided with a metal surface by vapor deposition are suitable in the process of the invention.

The thickness of the selfsupporting metal foils is not critical; it is primarily determined by the material properties, i.e., hardness, degree of brittleness, and elasticity. When electrophotographic materials of the invention are used which are not self-supporting, a metal layer of considerably less thickness may be employed.

Electrical properties may be influenced in known manner both during and after the oxidation process by alloying additions to the metal, e.g., nickel, iron or cobalt, or by the addition of foreign material to the electrolyte, e.g., sodium, copper or NH ions, or the oxide layer may be subjected to heat treatment, for example, in the presence of gases or oxides. This heat treatment is described in Reaktionen in und an festen Stofien," by Karl Hauffe, on page 225 et seq. (Springer-Verlag. l 5). a

For the preparation of copies using the clectrophotographic material of the invention, the photoelectrically conductive insulator coating, consisting of the oxide coating produced on a metal by anodization, is charged, for example, by a corona discharge from a charging apparatus maintained at 6000 to 7000 volts. The electrophotographic material is then placed in contact with a master and exposed, or a master is projected thereon episcopically or diascopically, whereupon an electrostatic image corresponding to the master is produced. This invisible image is developed by contact with a developer comprising a carrier and a toner. Suitable carriers are primarily fine glass balls, iron powder or fine plastic balls. The toner consists of a mixture of a resin and carbon black or a pigmented resin with an average grain size of about 1 to g. The developer may also consist of a resin or pigment suspended in a dielectric liquid. The image thus made visible is fixed in known manner, e.g., by heating with an infra-red radiator to about 100 to 170 C., preferably to C., or by treatment with solvents. Images are obtained which correspond to the master and which have good contrast effect.

After fixing, images obtained using the electrophotographic material of the invention can be employed as printing plates. The fixed image is made hydrophilic in known manner in the nonprinting parts, e.g., by wiping over with solutions of silicates or phosphates, and. after being inked up with greasy ink, can be used immediately in an offset printing machine. As there is no previous decoating operation to be performed this results in a considerable advantage over the elcctrophotographic material normally used. With printing plates prepared with electrophotographic material of the invention, printing images free of background giving very long printing runs are obtained.

The eleetrophotographic material of this invention has the advantage that it can be charged either positively or negatively, so that positive images can be obtained from negative and from positive masters with the same coating and the same developer simply by a reversal of polarity.

If, for example, the photoelectrically conductive insulator coating is negatively charged and a positive master is used for the exposure, positive images are obtained with a developer containing a positively charged toner. The positively charged toner settles on the parts not affected by the light, which are negatively charged.

With positive charging and conditions otherwise the same, positive images are obtained from negative masters. The positive toner is in this case repelled from the parts unaffected by the light, which are positively charged,

and settles on the exposed parts, which have been discharged.

it is also possible, using this clectrophotographie material, for the charge images obtained after imngcwise cxposure, or the powder images obtained after development, to be transferred in known manner to another support, e.g., paper.

' The invention will be further illustrated by reference to the following specific examples:

Example I An aluminum sheet having a thickness of 100p. was anodized in a 2.5% aqueous citric acid solution for minutes at C.; the voltage applied was volts. Aluminum was used as the counter-electrode. The oxidized aluminum sheet was dried at 100 to 150 C. for

twenty minutes; the oxide surface treated in this way i.e.,-

the photoelectrieally conductive insulator coating on the sheet material, was then negatively charged to about 18 to 20 volts. After the electrophotographie material thus treated had been exposed under a positive master to a high-pressure mercury vapor lamp, it was dusted over in known manner with a developer consisting of a mixture of a carrier and a toner. As the carrier, glass balls, iron powder and other inorganic or organic substances can be used. The toner consists of a resin/carbon black mixture'or of pigmented resins of a grain size between 1 and 100 An image corresponding to the master was formed, which was fixed by slight heating; it possessed good contrast efiect. f l

i Example II An aluminum foil with a thickness of 100 was anodized for 15 minutes at 200 volts and 20 C. in a 2.5% aqueous boric acid solution with a 0.05% borax addition. After being dried at 100 to 150 C., the oxide coating was positively charged by a corona discharge and exposed for seconds under a positive master to an ultra-violet quartz lamp at a distance of 30 cm. It was then dusted over with the developer described in Example 1. An image corresponding to the master, with clear contrast, became visible and was then fixed.

Example 111 An aluminum foil was anodically oxidized for 20 minutes at 250'volts in a 1% aqueous citric acid solution; an aluminum foil served as a counter electrode. 'The foil was then dried at--100 to 140 C. and the gleaming, matte-white oxide coating was positively charged and exposed for 40 seconds under a master to -a SOD-watt filament lamp at a distance of about 30 cm. The electrostatic charge image was then made visible with a resin powder such as that described in'Example 1. After the resin had been fixed at 150 to 190 C., the non-printing surfaces of the image were made hydrophilic by brief wiping over with a dilute solution of phosphoric acid and the plate then set up in an offset printing machine without further treatment. It was possible for a long run of background-free prints to be made. After the print ing, it was possible for the fixed resin coating to be re moved with ethyleneglycol monomethylether and for he aluminum foil to be used again several times as a printing plate in the same way.

Example IV mercury vapor lamp and dusted over in known manner with n developer consisting of a mixture of a carrier and n toner. As the carrier, inorganic substances were used and, as the toner, a resin/carbon black mixture of a grain size of between i and 100,11. An image corresponding to the master was formed and was fixed-by slight heating. It has very good contrast effect.

In the same way, it is also possible for the fixed tantalum sheet to be negatively charged up to volts and an image to be produced as mentioned.

Example V The procedure of Example II was followed. However, before the fixing process, the powder image was transferred by a known process, the application of a voltage, to a paper sheet placed thereon. Copies with good contrast effect are obtained.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

1. A photographic reproduction process which comprises exposing an electrostatically charged photoconductive insulating layer supported on a metal base to light under a master and developing the resulting image with an electroscopic material, the photoconductive layer comprising an oxide coating of the metal base produced by anodization.

2. A photographic reproduction process according to claim 1 in which the metal base is selected from the group consisting of aluminum, tantalum, and titanium.

3. A process for the production of a printing plate having a metal base which comprises exposing a photoconductive insulating layer, comprising an oxide coating of the metal base produced by anodizatior, to light under a master, developing the resulting image with an electroscopic material, fixing the developed image, and treating the metal base in the image-free areas thereof to render it hydrophilic.

4. A process according to claim 3 in which the metal base is selected from the group consisting of aluminum, tantalum, and titanium.

5. A photographic reproduction process according to claim 1 in which the metal base is aluminum. I

6. A photographic reproduction process according to claim I in which the metal base is tantalum.

7. A photographic reproduction process according to claim 1 in which the metal base is titanium.

8. A process for the production of a printing plate according to claim 3 in which the metal base is aluminum.

9. A process for the production of a printing plate according to claim 3 in which the metal base is tantalum.

10. A process for the production of a printing plate according to claim 3 in which the metal base is titanium.

References Cited in the file of this patent UNITED STATES PATENTS 2,681,310 Wood June 15, 1954 2,901,374 Gundlach Aug. 25, 1959 2,949,411 Beck Aug. 16, 1960 2,958,599 Neugebauer Nov. 1, 1960 3,082,085 Miller Mar. 19, 1963 FOREIGN PATENTS 201,301 Australia Mar. 19, 1956 OTHER REFERENCES Young: Transactions of the Faraday Society, vol. 50, pp. 153 and 164-171 (1954). TKLFZS. 

1. A PHOTOGRAPHIC REPRODUCTION PROCESS WHICH COMPRISES EXPOSING AN ELECTROSTATICALLY CHARGED PHOTOCONDUCTIVE INSULATING LAYER SUPPORTED ON A METAL BASE TO LIGHT UNDER A MASTER AND DEVELOPING THE RESULTNG IMAGE WITH AN ELECTROSCOPIC MATERIAL, THE PHOTOCONDUCTIVE LAYER COMPRISING AN OXIDE COATING OF THE METAL BASE PRODUCED BY ANODIZATION. 